Background: When the U.S. left Iraq, it was a country with a democratically-elected government and a U.S.-trained Army. But after the U.S. chased out Al Qaeda, remnants of the group reformed into a group called ISIS, and the democratically-elected leader, a Shiite, treated minority Sunnis and Kurds very poorly. ISIS gained the support of the Sunnis.

The group's initials stand for the Islamic State of Iraq and Syria; its goal is to create one unified Islamic state -- specifically, a Sunni one (This is important; we'll come back to it).

ISIS has taken over Mosul and Tikrit, major cities in the western part of Iraq populated mainly by Sunnis. They now control a huge swath of land in Syria and Iraq. They have declared their intention to take over Baghdad.

The official Iraqi government and army and have been unable to fight off ISIS. The stakes are high: Iraq could break apart into regions based on religion and ethnicity (Sunni, Shiite, Kurd), one group could gain control of the oil fields of Iraq, and the democratic process could be eliminated.

Reuters

PHOTO: Fighters of the Islamic State of Iraq and the Levant (ISIL) celebrate on vehicles taken from Iraqi security forces, at a street in city of Mosul, June 12, 2014.

Can You Remind Me About Sunnis, Shiites, and Kurds Quickly?

Yes. This is tricky but important. There are three main groups in Iraq: Sunni Arabs, Shiite Arabs and Sunni Kurds.

Sunni and Shiite are different sects of Islam, a religion.

Kurd is an ethnic group, like Arab or Indian or Caucasian.

In Iraq, the majority is Shiite Arabs who control the government. Then there are Sunni Kurds who live mostly by themselves in northern Iraq. And then there are Sunni Arabs who live in the western half of Iraq and are a political minority. Many Sunni Arabs are the ones joining and supporting the ISIS takeover.

AP Photo

PHOTO: Map locates cities in Iraq that were sites of attacks.

Why Do Sunnis Want an ISIS Takeover?

Many Sunni Muslims feel they were mistreated under the Shiite prime minister.

Since Shiite Muslims are the majority, they tend to elect Shiite leaders, so Sunni leaders may never be in top positions of power.

Because ISIS is Sunni, many Sunnis support them and see them as liberators.

Not all residents are supporting ISIS. Many who fear ISIS and combat have been forced to flee Mosul and Tikrit as ISIS has taken over.

AP Photo

PHOTO: Refugees fleeing from Mosul head to the self-ruled northern Kurdish region in Irbil, Iraq, June 12, 2014.